1. Field of Invention
This invention relates to a method and apparatus for making composition friction materials and particularly to a method and apparatus for making a composition from which brake shoes for the railroad industry may be readily produced.
2. Prior Art
Composition friction materials such as those used to make brake shoes and brake linings are commonly made by either a dry or wet-dry mix procedure.
In the dry mix method the various ingredients are combined and thoroughly mixed, as for example in a Lodige type mixer presently sold in the United States by Littleford Bros. Inc., Cincinnati, Ohio. The dry mixture so produced is then placed in a mold to which heat and pressure are applied to bind the ingredients into a physical friction unit.
The wet-dry mix method is the more commonly used to make composition friction materials for railroad brake shoes. Most such composition friction materials include an elastomer, i.e. a synthetic rubber such as butadiene-styrene, butadiene-acrylonitrile or chlor-butyl rubber, or a natural rubber which is mixed in the presence of a solvent, a resin base and friction materials such as powdered iron grit, lead sulfide and other fillers . After sufficient agitation and exposure to the solvent, the elastomer becomes sufficiently solvated or dissolved to allow dispersion therein of the balance of the fillers, resin curing agents and reinforcing fibers and to permit blending with a resin solution. This wet mix is then opened as in a hammermill and dried in trays in a circulating oven to remove substantially all of the solvent. The dry product then may be briquetted and then cured under heat and pressure in a mold to the desired shaped friction body.
The wet-dry procedure offers a number of distinct advantages over the dry mix procedure in the forming of composition friction materials for making railroad brake shoes.
Firstly, high percentage, heavy granular fillers such as iron grit, desirable in railroad brake shoes, remain dispersed and suspended throughout the mixture and is not gravitationally separated as occurs when using the dry-mix procedure. Secondly, the use of elastomeric materials has been found advantageous in producing a high performance railroad brake shoe and such material cannot be readily used in a dry mix procedure. Lastly, a resin binder, also a desirable ingredient, can only be effectively handled and utilized in the presence of a solvent. As used herein, any brake shoe mixture in which a solvent is used defines a wet-dry procedure.
U.S. Pat. No. 3,227,249 discloses a wet-dry mixing method for preparing a friction composition for use in making railroad brake shoes including generally the following steps:
1 A MIXTURE OF AN ELASTOMER, PARTIALLY CURED CASHEW NUT-SHELL OIL RESIN, REINFORCING FILLERS AND A CURING AGENT IS BLENDED IN A RUBBER MILL OR A BANBURY TYPE MIXER;
2 THE MIXTURE OF Step 1 is formed into a sheet;
3 the sheet stock is soaked in a solvent such as naptha or toluol in a closed container for a sufficient time period (24 hours) to swell the sheet mixture;
4 granular filler, friction modifiers and fibers are added to the swelled mixture;
5 the composite of Step 4 is mixed with an intensive mixer such as a sigma blade or dispersion blade mixer with a pressure lid to form a wet lumpy mix;
6 the mix of Step 5 is granulated with a hammermill to reduce the size of lumps; and
7 the mixture of Step 6 is dried in flat trays for proximately 15 hours at 150.degree. F. in an indirectly fired air oven to drive off the solvent.
The dried mix of Step 7 then may be formed into a briquette in any physical shape desired.